The overall objective of the present study is to develop a rational program of therapy for patients with recurrent paroxysmal supraventricular tachyarrhythmia (with and without preexcitation). To achieve this goal, we will detect, electrophysiologically study, and follow approximately 100 patients/year with one or more of the following arrhythmias; paroxysmal supraventricular tachycardia (PSVT), paroxysmal atrial fibrillation and paroxysmal atrial flutter. Atrial and ventricular stimulatory studies will be performed in regard to inducibility and mechanisms of the above arrhythmias. Chronic electrophysiological study (daily arrhythmia induction with a temporary electrode catheter) will be attemped in 50% of the patients, so that comparative pharmacologic data can be generated in regard to effect of multiple anti-arrhythmic drugs on tachyarrhythmic induction and mechanisms. In PSVT patients, we will attempt to determine ideal drugs in terms of depression of antegrade or retrograde limbs of circus movements. In patients with paroxysmal atrial fibrillation or flutter, we will attempt to demonstrate direct anti-fibrillatory and anti-flutter effects in the human atria. Drug responses will be correlatd with results of initial electrophysiological study, clinical parameters, and surface electrocardiographic findings (during sinus rhythm and tachyarrhythmia). All patients will be followed in research clinics, so that the predictive value of initial and chronic electrophysiological study can be evaluated. Dr. Lev's laboratory will perform serial section of the conduction system in patients dying during follow-up, in attempt to achieve electrocardiographic and electrophysiological-pathological correlations. We will attempt to replace current trial and error management of these dysrhythmia with a rational therapeutic program, ultimately based upon single electrophysiological study, or ideally, from just clinical and/or electrocardiographic findings.